This invention discloses means for purging oil from engine hot sections after shutdown so that coking does not occur as a result of heat soakback.
Higher specific power and improved cycle efficiency in gas turbine engines result from operating the turbine section at higher temperatures. This is basic to the nature of the Brayton cycle. Cooling techniques used on large engines do not lend themselves to easy scaling to small turbines. This results from an inability to cast or machine proportionately scaled internal cooling geometry due to minimum wall thickness requirements and an inability to reduce leakages due to seal clearance and assembly tolerance limitations.
In consequence, a small turbine engine that has been designed for low specific fuel consumption, will experience different temperature problems in the turbine section than will a similar large engine. When the turbine is shut down from a high power condition, there occurs a condition known as heat soakback. This results from the heat residing in the hottest engine sections being gradually transferred to the cooler parts of the engine through conduction, convection and radiation. During operation both air and oil cooling are used to keep operating temperature under control. After shutdown, heat is lost only through radiation and convection from the exterior surfaces of the engine. Any oil remaining in the jets or passages of the engine during the heat soakback period will be heated to the temperature of the surrounding metal. If the temperature of the oil rises to values in excess of 500 degrees Fahrenheit, coking occurs. In a small engine coking becomes a problem since the orifices at the oil jets are small. If coking occurs, the bearings and seals which the jets supply with lubricating oil tend to be starved when the engine is restarted. Lubricant starvation results in premature bearing and seal failure.
Our invention overcomes this problem in that in critical areas, both the oil lines and the jets are purged of oil each time the engine is shut down. Purging is accomplished automatically some 15 to 30 seconds after shutdown.